1. Field
Example embodiments relate to a multi-well CMOS (complimentary metal oxide semiconductor) image sensor and a method of fabricating the same.
2. Description of the Related Art
An image sensor is a photoelectric element that transforms detected light into an electrical signal. A conventional image sensor may include a plurality of unit pixels arranged in an array on a semiconductor substrate. Each of the unit pixels may include a photodiode and transistors. When detecting external light, the photodiode may generate and store optical charges and the transistors may output electrical signals according to the charges of the generated optical charges.
A CMOS image sensor may include a photodiode that may store received optical signals and may realize an image using a control device that may control or process optical signals. Because the control device may be manufactured using a CMOS manufacturing technique, the process of manufacturing the CMOS image sensor may be relatively simple, and furthermore, may be formed in one chip including various signal processing devices.
In a conventional CMOS image sensor, the photodiode, which receives light that has passed through a micro-lens and a color filter, may be a pn junction diode. In response to the received light, the photodiode may generate pairs of electrons and holes, and electrons may accumulate in a well due to a potential well. The accumulated electrons may move to a floating diffusion region when a transfer gate is opened, and a voltage may be outputted from the floating diffusion region according to the amount of electrons.
However, the potential well of the photodiode of the conventional CMOS image sensor may be formed to a depth of 0.1 to 1.5 μm from a silicon surface. The practical depth at which light is absorbed in silicon may be different according to the type of light. Blue light may be mostly absorbed on the surface, and the absorption depth may increase as the wavelength of light becomes shorter. In the case of red light, about half of the incident light may be absorbed to a depth of 3 μm. Therefore, if the potential well is formed to a higher depth, the sensitivity of the CMOS image sensor may be increased.
FIG. 1 illustrates a simulation result of a potential energy of a conventional CMOS image sensor. For convenience, a photodiode of the image sensor and a floating diffusion region are depicted only. Referring to FIG. 1, the photodiode, which is an n type potential well, may be formed to a depth of 0.1 to 1.5 μm from a surface of a p type silicon substrate. The contour lines may indicate identical concentration, and the concentration of the n type impurity may increase towards the center. A floating diffusion region, which is an n+ doping region, may be formed on a side of the photodiode. Thus, when a transfer gate is opened, electrons from the photodiode may move to the floating diffusion region where the potential is lower. A first graph G1 indicates the potential of the potential well of FIG. 1, and a second graph G2 indicates the potential when the potential well may be formed to a depth of 2 μm. The potential of the first and second graphs G1 and G2 may increase towards the right-hand side. At the second graph G2 where a relatively deep potential well is formed, the bottom of the potential well may become flat, and thus, electrons and holes formed due to optical absorption may be recombined instead of being separated, so the sensitivity of the image sensor may be reduced.